Saccharomyces cerevisiae comprises various strains, of which each has differing properties. So, there are many strains which had previously been reported as different species from Saccharomyces cerevisiae, but then were determined to belong to this species (C. P. Kurtzman and J. W. Fell: The Yeasts, A taxonomic study, 4th ed. Elsevier, The Netherlands, 1998). Since strains belonging to Saccharomyces cerevisiae are frequently used for the production of various fermented food or alcoholic beverages, the instability of the classification system of Saccharomyces cerevisiae strains presents problems, causing confusion at the site of study or production.
Although a subclassification on the basis of the requirement of vitamins has been carried out up till now (Tatsuyoshi Yamaguchi: “A study regarding the classification of baker's yeast (the 6th publication), the classification of baker's yeast in each country”, Biosci. Biotech. Biochem., vol.33, p. 513-519, 1959), the analysis of the strains was difficult.
In recent years, however, the development of molecular phylogeny mainly regarding bacteria, which uses ribosomal RNA molecules, has allowed microorganisms including bacteria to be phylogenetically classified. Yeast belonging to fungus can also be phylogenetically classified by making a comparison of cytoplasmic ribosomal RNA or the nucleotide sequence of the gene. However, since there are very few mutations in the nucleotide sequences of cytoplasmic ribosomal RNAs, it is considered that the cytoplasmic ribosomal RNA is not suitable for the phyloanalysis of allied strains.